Light-emitting devices such as vertical-cavity surface-emitting lasers (VCSELs) are known in the art. The active region of a VCSEL has a quantum well structure composed of one or more quantum well layers interleaved with a corresponding number of barrier layers. Each quantum well layer forms a quantum well with the adjacent barrier layers. The quantum well captures and confines carriers (electrons and holes), which subsequently radiatively recombine to generate light.
The quantum well structure of a conventional VCSEL that generates light at a wavelength of 980 nm can have quantum well layers of indium gallium arsenide (InGaAs) and barrier layers of gallium arsenide (GaAs), or have quantum well layers of InGaAs and barrier layers of gallium arsenide phosphide (GaAsP), or have quantum well layers of InGaAs and barrier layers of aluminum gallium arsenide (AlGaAs). However, there are disadvantages associated with each of these approaches.
For example, conventional InGaAs/GaAs quantum well structures (where the InGaAs has an alloy composition of 20% In) provide insufficient confinement of the carriers, which can adversely affect the higher modulation speed of the typical VCSEL. One conventional way the confinement of carriers may be improved in the InGaAs/GaAs quantum well structures is to increase the amount of In within the quantum wells and reduce the quantum well thickness in order to keep the same emission wavelength. However, this approach adds additional strain to the system, which may negatively impact its reliability. Furthermore, the additional strain introduced in these structures also increases the splitting between the light hole and heavy hole bands, which can lead to increased intra-valence band scattering of carriers which in turn adversely affects modulation speed.
Conventional InGaAs/GaAsP quantum well structures typically have two disadvantages associated with them. For example, the interface between the GaAsP quantum well layer and the InGaAs barrier layer can become non-abrupt due to alloy intermixing introduced by different group V compositions. Additionally, the tensile strained GaAsP quantum well layers can have mid gap states that lead to lower confinement of the carriers than predicted by a simple band lineup model.
The InGaAs/AlGaAs quantum well structures have a disadvantage associated with them. For example, the InGaAs/AlGaAs quantum well structures have deeper quantum wells that contribute to non-uniform carrier distribution, especially for holes which have higher effective mass than the electrons.